DESCRIPTION: With the thinning of the ozone layer, individuals are exposed to increasing levels of UV irradiation. A clear consequence of increased exposure to UV light is skin cancer, both non-melanoma and melanoma. All cancers occur as a result of damage to the cell's DNA, leading to the loss of suppressor oncogene activity or the activation of dominant oncogenes. A cell's ability to escape cancer following DNA damage is largely dependent on its ability to correctly repair its damage. Recently, the principal investigator has obtained genetic and biochemical data which suggests that there is a new DNA excision repair process in the fission yeast Schizisacchromyces pombe. The key component of this excision repair process is a UV photoproduct endonuclease, SPDE for S. pombe DNA endonuclease. SPDE cleaves 5' to the major cytotoxic and mutagenic UV photoproducts, cyclobutane pyrimidine dimers and (6-4) pyrimidine-pyrimidone adducts, as the initial recognition/excision step of this excision repair process. The goal of this proposal is to study the mechanism of SPDE-dependent DNA repair using an in vitro assay. SPDE protein will be purified and characterized along with other components of this repair process. The gene for SPDE will be isolated and a possible human homologue identified. It is hypothesized that SPDE-dependent DNA repair is an important component in preventing UV induced skin cancers.